Field of the Invention
The present invention relates to an optical information recording medium provided with both a read-only-memory (ROM) region and a recording region.
Discussion of Background
Conventional recordable type information recording media, in which information can be recorded by application of a laser beam thereto, comprise a recording layer such as a metal layer comprising Te, Bi or Mn, or a dye layer comprising a cyanine dye, a melocyanine dye or a phthalocyanine dye. Information is recorded in such recording media by the deformation, sublimation or evaporation of the recording layer, or by changing the properties of the recording layer, with application of a laser beam thereto.
On the other hand, conventional ROM type optical information recording media, in which information has been recorded, and no additional data can be recorded afterwards or the recorded information cannot be erased, comprise (i) a substrate made of, for example, polycarbonate, in which pits or pregrooves for reproducing recorded information have been formed by a pressing process, (ii) a reflective layer such as a metal layer made of Au, Ag, Cu, Al or the like, which is provided on the substrate, and (iii) a protective layer provided on the reflective layer. Representative examples of such ROM type optical information recording media are compact disks.
The ROM type optical information recording media with an identical information or data recorded therein can be mass produced, and there is no risk that the recorded information or data is erased by mistake. However, the user cannot additionally record his or her desired information in the recording medium.
On the other hand, in the recordable type optical information recording media, the user can record information as desired, but there is the risk that the recorded information is erased by mistake. Furthermore, the recordable type optical information recording media are not suitable for mass producing recording media with an identical information being recorded.
Because of these respective advantages and disadvantages of the two types of optical information recording media and the respective limited application fields thereof, an optical information recording medium 11 including both a ROM region 12 and a recording region 13 as shown in FIG. 4 has been proposed and disclosed in Japanese Laid-Open Patent Application 2-42652. The optical information recording medium 11 can be obtained by coating a recording layer 15, in the recording region 13, on a substrate 14 on which a pit 18 or a pregroove 19 has been formed, providing a reflective layer 16 such as a metallic layer so as to cover both the ROM region 12 and the recording layer 15 in the recording region 13, and finally providing a protective layer 17 on the reflective layer 16 in its entirety as shown in FIG. 4.
However, in the method of producing the above recording medium, the recording layer 15 is first formed by spin coating in the recording region 13 of the substrate 14. If the viscosity of the coating liquid for the formation of the recording layer 15 is decreased to make the thickness of the recording layer 15 uniform, the coating liquid for the recording layer 15 enters the ROM region 12. The result is that the boundary between the ROM region 12 and the recording region 13 does not become concentric.
In order to prevent such a problem, a method of shielding the ROM region 12 with a shielding plate (not shown) has been proposed. However, when the viscosity of the coating liquid is low, the coating liquid enters between the shielding plate and the substrate 14 and when the spin coating method is employed, the coating liquid particles collide with the shielding plate and then return onto the recording layer 15. As a result, the thickness of the recording layer 15 is slightly changed from place to place, so that a recording layer with a uniform thickness cannot be obtained. When the recording layer 15 does not have a uniform thickness, the laser beams reflected by the recording layer 15 are made non-uniform so that the recording characteristics of the recording medium, such as the servo performance including tracking performance and focusing performance, are impaired.
Furthermore, when the above-mentioned shielding plate is employed, the contact of the shielding plate with the substrate 14, or the gap between the shielding plate and the substrate 14 must be precisely controlled in order to prevent the substrate 14 from being scratched by the shielding plate. In addition, the utmost care must be taken to prevent the shielding plate from being scratched or smeared.
When a plurality of such ROM regions and recording regions is to be provided, it is extremely difficult to provide the recording layer 15 by using such shielding plates.